Bio-imaging techniques can non-invasively measure biological functions, evaluate cellular and molecular events, and reveal the inner workings of a body. Examples of bio-imaging techniques include magnetic resonance imaging (MRI), positron emission tomography (PET), x-ray tomography, luminescence (optical imaging), and ultrasound. Each of these techniques can differ from one another in the resolution, sensitivity, and anatomical information they provide about the subject. For example, though optical imaging has high sensitivity, it provides limited anatomical background information, and can display artifacts due to tissue absorbance and scattering. Photo acoustic tomography (PAT), an emerging non-invasive imaging modality, uses a non-ionizing optical (pulsed laser) source to generate contrast, which is detected as an acoustic signal whose scattering is 2-3 orders of magnitude weaker than optical scattering in biological tissues, the primary limitation of optical imaging. MRI on the other hand can be used to generate contrast to detect tumors in deep tissue and provide true three-dimensional imaging of biological structures and processes at cellular resolution. X-ray contrast is useful to differentiate tissues with small differences in their opacity.
It is often necessary to use more than one imaging technique to integrate the strengths of each while overcoming the limitations of the individual techniques to improve diagnostics, preclinical research and therapeutic monitoring. However, each of these techniques typically uses a different contrast agent, so using more than one bio-imaging technique requires additional time, expense and can complicate the diagnostic process. It would be desirable to have a multimodal contrast agent that can be used for more than one bio-imaging technique. Multi-modal contrast agents for bioimaging can also serve as important tools for developing and benchmarking experimental imaging technologies by using parallel experiments with mature, proven technologies. The application of multimodal contrast agents is particularly important for developing less expensive, more available, and reliable bio-imaging technologies, such as PAT, that have the potential to make advanced medical diagnostics available to impoverished populations, as well as more commonplace worldwide. Although imaging technologies such as CT and MRI have become prevalent, the related capital costs associated with obtaining and maintaining existing equipment financially limits its widespread application, even in modern societies.